Changes to `tag:misd.isi.edu,2015:url` specification
Changes to `tag:misd.isi.edu,2015:url` specification

Summary of changes:
- the annotation can be applied to tables as well as columns
- Dropped baseUrl
- Change "pattern" to "url" that always takes a pattern
- The pattern can take multiple columns (see below)
- Added "presentation" which takes options on how to present it
  - "embed" is one of its option to indicate use in iframe
  - "thumbnail" now goes here, and is dropped as a top level option
- Added "caption" which is also based on a pattern
- The payload body should support not just a single object but an array of
the objects.

informatics-isi-edu/ermrest,ini-bdds/ermrest,ini-bdds/ermrest,informatics-isi-edu/ermrest